Aluminum is wonderful stuff, but it's almost completely transparent to magnetic fields.
Steel is cheaper, even when plated with nickel for corrosion resistance, and is effective against both magnetic and electric fields.
The third alternative that will inevitably come up is plastic coated with metallic particles, by means of metal spraying or paint. The problem with that scheme, in addition to the horrendous expense of conductive paint, is that it's impossible to ground it reliably.
In order to be effective against both E and H fields, the metallic shield surrounding the electronics must be almost completely continous, and grounded. The allowable gap size is dependent on the frequency involved, and is now down to a couple of millimeters, thanks to cheap fast computer chips.
For a really good example of effective contemporary shielding, open the case of a PowerMac G3 or similar. Inside the plastic case you will find a nickel plated steel shell, with teeth lining every edge. Be careful; the steel is thin and the edges are razor sharp.
Mike Halloran
Pembroke Pines, FL, USA